docs: def sql(self):
          """
          Convert primary key attribute tuple into its SQL CREATE TABLE clause.
          Default values are not reflected.
          This is used for declaring foreign keys in referencing tables
          :return: SQL code
          """
          assert self.in_key and not self.nullable   # primary key attributes are never nullable
          return '`{name}` {type} NOT NULL COMMENT "{comment}"'.format(
              name=self.name, type=self.type, comment=self.comment)